Pre-sintered blank for dental purposes

ABSTRACT

Pre-sintered blanks based on lithium metasilicate glass ceramic are described which are suitable in particular for the preparation of dental restorations.

The invention relates to a pre-sintered blank for dental purposes basedon lithium metasilicate glass ceramic which is suitable in particularfor the preparation of dental restorations.

Reports on the use of pre-sintered blanks in dentistry have already beenmade in the state of the art.

WO 2010/010087 describes porous silicate-ceramic shaped bodies which areprocessed to form veneers for dentistry. The shaped bodies should have aparticular density in order to prevent damage during the machining withmilling or grinding systems, e.g. due to the material bursting, andshould be suitable for the selected system.

U.S. Pat. No. 5,106,303 describes the preparation of tooth crowns andinlays by copy milling of compacted ceramic bodies which can optionallybe pre-sintered. To achieve the desired geometry, the bodies are milledto an enlarged shape in order to take into consideration the shrinkagethat occurs during the subsequent sintering to the desired high density.Aluminium oxide, which can optionally include strengthening additives,is used in particular as ceramic material.

U.S. Pat. No. 5,775,912 describes pre-sintered dental porcelain pellets,from which a tooth structure is milled by means of CAD/CAM systems. Thistooth structure is embedded in embedding material, sintered and removedfrom the embedding material in order to produce the desired dentalrestoration. The dental porcelains used are glass ceramics based onleucite.

U.S. Pat. No. 6,354,836 discloses methods of manufacturing dentalrestorations using CAD/CAM methods. For this, unsintered or pre-sinteredblocks of ceramic material and in particular aluminium oxide andzirconium oxide are used which result in high-strength dentalrestorations after milling to an enlarged shape followed by densesintering. However, it is considered to be essential that thetemperature differences in the sintering furnace used are smaller than10° C. in order to ensure that variations in the finally achieveddimensions of the restorations are small.

With the known pre-sintered blanks, the shrinkage occurring during thedense sintering and thus the enlargement factor to be applied depends toa great extent on the pre-sintering temperature applied. Even smallvariations, such as can occur as a result of an inhomogeneoustemperature distribution in the sintering furnace, result in differentshrinkages during the dense sintering. However, these shrinkages do notallow the desired small tolerances in the dimensions of the produceddental restoration.

The object of the invention is therefore to provide blanks which avoidthese disadvantages and are therefore less susceptible to variations inthe sintering temperature applied for their preparation. Likewise, theseblanks should be able to be shaped easily by means of customary grindingand milling processes to form dental restorations with the desiredgeometry, without liquid needing to be supplied during these processes.Furthermore, these blanks should be able to be processed by densesintering to form high-strength and optically very attractive dentalrestorations.

This object is achieved by the pre-sintered blank according to claims 1to 9. Another subject of the invention is the process for thepreparation of the blank according to claims 10 and 11, the process forthe preparation of dental restorations according to claims 12 to 15 aswell as the use of the blank according to claim 16.

The pre-sintered blank according to the invention for dental purposes ischaracterized in that it

-   -   is based on lithium metasilicate glass ceramic and    -   has a relative density of from 66 to 90%, in particular 68 to        88% and preferably 70 to 86%, relative to the true density of        the corresponding dense-sintered lithium disilicate glass        ceramic.

The relative density is the ratio of the density of the pre-sinteredblank to the true density of the corresponding dense-sintered lithiumdisilicate glass ceramic.

The density of the pre-sintered blank is determined by weighing it andascertaining its volume geometrically. The density is then calculatedaccording to the known formula

density=mass/volume.

The true density of the corresponding dense-sintered lithium disilicateglass ceramic is determined by heat-treating the pre-sintered blank for20 min in a furnace heated to 920° C., grinding the obtainedcorresponding dense-sintered lithium disilicate glass ceramic to apowder with an average particle size of from 10 to 30 μm, in particularof 20 μm, relative to the number of particles and ascertaining thedensity of the powder by means of a pycnometer. The determination of theparticle size was carried out by means of laser diffraction inaccordance with ISO 13320 (2009) with the CILAS® Particle Size Analyzer1064 from Quantachrome GmbH & Co. KG.

It has surprisingly been found out that not only can the blank accordingto the invention be machined dry in a simple way, but it can also beprepared at significantly different pre-sintering temperatures, withoutthis resulting in a substantial change in the shrinkage which occursduring a subsequent dense sintering. The enlargement factor taking intoconsideration the shrinkage that occurs can thus be determined veryprecisely. These advantageous properties are clearly to be attributed tothe particular behaviour of lithium metasilicate glass ceramic duringthe pre-sintering to the relative densities given above and its abilityto be converted to high-strength lithium disilicate glass ceramic athigh temperatures, such as are customarily applied e.g. during the densesintering.

It is further preferred that the blank consists substantially of lithiummetasilicate glass ceramic. Particularly preferably, the blank consistsof lithium metasilicate glass ceramic.

The glass ceramic includes lithium metasilicate as main crystal phase ina preferred embodiment. The term “main crystal phase” denotes thecrystal phase which has the highest proportion by volume compared withother crystal phases. In particular the glass ceramic contains more than20 vol.-%, preferably more than 25 vol.-% and particularly preferablymore than 30 vol.-% lithium metasilicate crystals, relative to the totalglass ceramic.

The lithium metasilicate glass ceramic contains SiO₂ and Li₂O,preferably in a molar ratio in the range of from 1.75 to 3.0, inparticular 1.8 to 2.6 and particularly preferably 2.2 to 2.5.

In a further preferred embodiment, the lithium metasilicate glassceramic contains at least one of the following components:

Component wt.-% SiO₂ 50.0 to 80.0 Li₂O 6.0 to 20.0 Me(I)₂O 0 to 10.0, inparticular 0.1 to 10.0 Me(II)O 0 to 12.0, in particular 0.1 to 12.0Me(III)₂O₃ 0 to 8.0, in particular 0.1 to 8.0 Me(IV)O₂ 0 to 8.0, inparticular 0.1 to 8.0 Me(V)₂O₅ 0 to 8.0, in particular 0.1 to 8.0Me(VI)O₃ 0 to 8.0, in particular 0.1 to 8.0 nucleating agent 0 to 8.0,in particular 0.1 to 8.0

-   -   wherein    -   Me(I)₂O is selected from Na₂O, K₂O, Rb₂O, Cs₂O or mixtures        thereof,    -   Me(II)O is selected from CaO, BaO, MgO, SrO, ZnO and mixtures        thereof,    -   Me(III)₂O₃ is selected from Al₂O₃, La₂O₃, Bi₂O₃, Y₂O₃, Yb₂O₃ and        mixtures thereof,    -   Me(IV)O₂ is selected from ZrO₂, TiO₂, SnO₂, GeO₂ and mixtures        thereof,    -   Me (V)₂O₅ is selected from Ta₂O₅, Nb₂O₅, V₂O₅ and mixtures        thereof,    -   Me(VI)O₃ is selected from WO₃, MoO₃ and mixtures thereof, and    -   nucleating agent is selected from P₂O₅, metals and mixtures        thereof.        Na₂O and K₂O are preferred as oxides of monovalent elements        Me(I)₂O.        CaO, MgO, SrO and ZnO are preferred as oxides of divalent        elements Me(II)O.        Al₂O₃, La₂O₃ and Y₂O₃ are preferred as oxides of trivalent        elements Me(III)₂O₃.        ZrO₂, TiO₂ and GeO₂ are preferred as oxides of tetravalent        elements Me(IV)O₂.        Ta₂O₅ and Nb₂O₅ are preferred as oxides of pentavalent elements        Me(V)₂O₅.        WO₃ and MoO₃ are preferred as oxides of hexavalent elements        Me(VI)O₃.        P₂O₅ is preferred as nucleating agent.

The lithium metasilicate glass ceramic preferably contains colorantsand/or fluorescent agents.

Examples of colorants and fluorescent agents are oxides of d- andf-elements, such as the oxides of Ti, V, Sc, Mn, Fe, Co, Ta, W, Ce, Pr,Nd, Tb, Er, Dy, Gd, Eu and Yb, and ceramic pigments, such as colouredspinels. Metal colloids, e.g. of Ag, Au and Pd, can also be used ascolorants and in addition can also act as nucleating agents. These metalcolloids can be formed e.g. by reduction of corresponding oxides,chlorides or nitrates during the melting and crystallization processes.

The blank according to the invention preferably has at least two areas,in particular layers, which differ in terms of their coloration ortranslucence. The blank preferably has at least 3 and up to 10,particularly preferably at least 3 and up to 8, and even more preferablyat least 4 and up to 6 areas, in particular layers, differing incoloration or translucence. The imitation of natural tooth material isvery successful precisely because of the presence of several differentlycoloured areas, in particular layers. It is also possible that at leastone of the areas or of the layers has a colour gradient to ensure acontinuous colour transition.

In a further preferred embodiment, the blank according to the inventionhas a holder for securing it in a processing device. In anotherpreferred embodiment, the blank according to the invention has aninterface for connection to a dental implant.

The holder allows the blank to be secured in a processing device, suchas in particular a milling or grinding device. The holder is usually inthe form of a pin and preferably consists of metal or plastic.

The interface ensures a connection between an implant and the dentalrestoration fitted thereon, such as in particular an abutment crown,which has been obtained by machining and dense sintering of the blank.This connection is preferably rotationally fixed. The interface ispresent in particular in the form of a recess, such as a bore. Thespecific geometry of the interface is usually chosen depending on theimplant system used in each case.

The invention also relates to a process for the preparation of the blankaccording to the invention, in which

-   (a) lithium silicate glass in powder or granulate form is pressed to    form a glass blank,-   (b) the glass blank is heat-treated in order to prepare a    pre-sintered blank based on lithium metasilicate glass ceramic,    wherein the temperature of the heat treatment    -   (i) is at least 500° C., in particular at least 540° C. and        preferably at least 580° C., and    -   (ii) lies in a range which extends over at least 30K, in        particular at least 50K and preferably at least 70K and in which        the relative density varies by less than 2.5%, in particular        less than 2.0% and preferably less than 1.5%.

In stage (a), lithium silicate glass in powder or granulate form ispressed to form a glass blank.

The lithium silicate glass employed is usually prepared by melting amixture of suitable starting materials, such as carbonates, oxides,phosphates and fluorides, for 2 to 10 h at temperatures of in particularfrom 1300 to 1600° C. To achieve a particularly high homogeneity, theobtained glass melt is poured into water in order to form a glassgranulate, and the obtained granulate is then melted again.

The granulate is then comminuted to the desired particle size and inparticular ground to powder with an average particle size of <50 μm,relative to the number of particles.

The granulate or powder is then, optionally together with added pressingauxiliaries or binders, usually placed in a compression mould andpressed to form a glass blank. The pressure applied lies in particularin the range of from 20 to 200 MPa. Uniaxial presses are preferably usedfor the pressing. The pressing can in particular also be isostaticpressing, preferably cold isostatic pressing.

Through the use of glass powders or glass granulates with differentcoloration or translucence, glass blanks can be produced which havedifferently coloured or differently translucent areas and in particularlayers. For example, differently coloured powders or granulates can bearranged on top of one another in a compression mould, with the resultthat a multi-coloured glass blank is produced. The multiple colours makeit possible to a great extent to give the finally prepared dentalrestorations the appearance of natural tooth material.

In stage (b), the obtained uni- or multi-coloured glass blank issubjected to a heat treatment in order to bring about the pre-sinteringand controlled crystallization of lithium metasilicate and thus theformation of lithium metasilicate glass ceramic. The heat treatmenttakes place in particular at a temperature of from 500 to 800° C.,preferably from 540 to 800° C. and particularly preferably from 580 to750° C. The heat treatment is carried out in particular for a period offrom 5 to 60 min, preferably 10 to 40 min and particularly preferably 15to 30 min.

The temperature range (b)(ii) describes a range in which, despite achange in temperature, the relative density hardly changes. This rangeis therefore also referred to as “plateau” in the following. Thevariation in the relative density possible in this range is calculatedin % from the maximum and minimum value of the relative density in therange by

(maximum value−minimum value)/maximum value×100

It has surprisingly been shown that during the pre-sintering inparticular temperature ranges lithium metasilicate glass ceramicsdisplay essentially no change in the relative density and thus in thelinear shrinkage and the enlargement factor during the dense sintering.These ranges are recognizable as “plateaus” in the graphicrepresentation of relative density, linear shrinkage or enlargementfactor against the temperature. Accordingly, properties of the blankthat are important for the accuracy of fit of the later dentalrestoration are essentially not dependent on the temperature in thisrange. The result of this is the important practical advantage that theblank tends to be unsusceptible e.g. to temperature fluctuations ortemperature gradients in the sintering furnace, as long as thetemperature is in the “plateau” range.

According to the invention, therefore, pre-sintered blanks which havebeen prepared using the process according to the invention areparticularly preferred.

Particularly preferred are blanks according to the invention which havea relative density which results when

-   (a) powder of a corresponding starting glass with an average    particle size of <50 μm, relative to the number of particles, is    uniaxially or isostatically pressed at a pressure of from 20 to 200    MPa, preferably 40 to 120 MPa and particularly preferably 50 to 100    MPa and-   (b) the obtained glass powder green compact is heat-treated for to    60 min, preferably 10 to 40 min and particularly preferably 15 to 30    min at a temperature which    -   (i) is at least 500° C., in particular at least 540° C. and        preferably at least 580° C., and    -   (ii) lies in a range which extends over at least 30K, in        particular at least 50K and preferably at least 70K and in which        the relative density varies by less than 2.5%, in particular        less than 2.0% and preferably less than 1.5%.

FIG. 2 illustrates the phases usually passed through during heattreatment of a glass powder green compact by plotting the enlargementfactor against the temperature for a green compact with a compositionaccording to Example 1. In Phase I, up to about 400° C., the heating andthe removal of any binder present take place. In Phase II, from about400 to about 600° C., sintering and crystallization take place, and inPhase III, the plateau, from about 600 to about 700° C., there is apre-sintered blank according to the invention based on lithiummetasilicate glass ceramic. Then, in Phase IV, starting from about 700°C., the dense sintering and crystallization of lithium disilicate takeplace.

The pre-sintered blank according to the invention is preferably presentin the form of blocks, disks or cylinders. In these forms, a furtherprocessing to form the desired dental restorations is particularly easy.

The pre-sintered blank is further processed in particular to form dentalrestorations. The invention therefore also relates to a process for thepreparation of dental restorations, in which

-   (i) the pre-sintered blank according to the invention based on    lithium metasilicate glass ceramic is shaped by machining to form a    precursor of the dental restoration,-   (ii) the precursor is substantially dense sintered in order to    produce the dental restoration, and-   (iii) optionally the surface of the dental restoration is provided    with a finish.

In stage (i), the machining is usually carried out by material removalprocesses and in particular by milling and/or grinding. It is preferredthat the machining is carried out with computer-controlled millingand/or grinding devices. Particularly preferably, the mechanical workingis carried out as a step of a CAD/CAM process.

The blank according to the invention can be machined very easily inparticular because it is open-pored and has low strength. It isparticularly advantageous that it is not necessary to use liquids duringthe grinding or milling. In contrast to this, so-called wet-grindingprocesses are often necessary with conventional blanks.

The machining is usually carried out in such a way that the obtainedprecursor represents an enlarged form of the desired dental restoration.The shrinkage occurring during the subsequent dense sintering is therebytaken into consideration. The blank according to the invention has theparticular advantage that the enlargement factor to be applied to it canbe determined very precisely. The enlargement factor is the factor bywhich the precursor has to be ground or milled enlarged out of thepre-sintered blank in order that after the dense sintering the obtaineddental restoration has the desired dimensions.

The enlargement factor F_(v), the relative density ρ_(r) and theremaining linear shrinkage S can be converted into each other asfollows:

S=1−ρ_(r) ^(1/3)

F _(v)=1/(1−S)

In a preferred embodiment, the blank produced according to theabove-described process according to the invention is used aspre-sintered blank.

In stage (ii) the obtained precursor is substantially dense-sintered inorder to produce the dental restoration with the desired geometry.

For the dense sintering, the precursor is preferably heat-treated at atemperature of from 800 to 1000° C., in particular from 850 to 950° C.The heat treatment usually takes place for a period of from 2 to 40 min,in particular 2 to 30 min and particularly preferably 5 to 15 min.During this heat treatment, not only does a dense sintering take place,but also usually the conversion of the lithium metasilicate glassceramic into lithium disilicate glass ceramic.

There is then a dental restoration based on lithium disilicate glassceramic. In this glass ceramic, lithium disilicate preferably forms themain crystal phase. This lithium disilicate glass ceramic has excellentoptical and mechanical properties as well as a high chemical stability.Dental restorations which meet the high demands made on them can thus beprepared with the process according to the invention.

The dental restorations are preferably selected from crowns, abutments,abutment crowns, inlays, onlays, veneers, shells and bridges as well asoverstructures for multi-part restoration frames which can consist e.g.of oxide ceramic, metals or dental alloys.

It can be advantageous for the dense sintering that the precursor of thedental restoration is supported in order to avoid a distortion. It ispreferred that the support consists of the same material as theprecursor and hence shows the same shrinkage upon sintering. The supportcan be in form of for example a supporting structure or supporting mouldwhich in terms of their geometry are adapted to the precursor.

In the optional stage (iii), the surface of the dental restoration canalso be provided with a finish. It is possible in particular to alsocarry out a glazing firing at a temperature of from 700 to 900° C. or topolish the dental restoration.

Because of the described properties of the pre-sintered blank accordingto the invention, it is suitable in particular for producing dentalrestorations. The invention therefore also relates to the use of theblank to prepare dental restorations and in particular crowns,abutments, abutment crowns, inlays, onlays, veneers, shells and bridgesas well as overstructures.

The average particle sizes given, relative to the number of particles,were determined by laser diffraction with the CILAS® Particle SizeAnalyzer 1064 from Quantachrome GmbH & Co. KG in accordance with ISO13320 (2009).

The invention is explained in more detail below by means of examples.

EXAMPLES Examples 1 to 16

A total of 16 glass ceramics with lithium metasilicate as main crystalphase with the compositions given in Table I were prepared by meltingcorresponding starting glasses and then pre-sintering, by heattreatment, pressed glass powder blanks produced from them and at thesame time crystallizing lithium metasilicate.

For this purpose, the starting glasses on a scale of 100 to 200 g werefirst melted from customary raw materials at 1400 to 1500° C., whereinthe melting could be carried out very easily without formation ofbubbles or streaks. By pouring the starting glasses into water, glassfrits were prepared which were then melted a second time at 1450 to1550° C. for 1 to 3 h for homogenization.

The obtained glass melts were then cooled to 1400° C. and converted tofine-particle granulates by pouring into water. The granulates weredried and ground to powder with an average particle size of <100 μm,relative to the number of particles. These powders were moistened in aspray drier with a binder customary in the trade and then pressed toform powder green compacts at a pressing pressure of from 20 to 200 MPa.

The powder green compacts were then heat-treated for 2 to 120 min at atemperature which lies in the range given as plateau in Table I for therespective composition. After this heat treatment, blanks according tothe invention were present which were pre-sintered and based on lithiummetasilicate glass ceramic.

TABLE I Example 1 2 3 4 5 6 7 8 Component wt.-% wt.-% wt.-% wt.-% wt.-%wt.-% wt.-% wt.-% SiO2 71.15  69.76  69.20  71.02  72.09  71.39  71.52 68.43  K2O 4.20 3.70 3.80 4.29 4.24 4.20 4.21 4.22 SrO — — — 0.99 — 1.00— 1.56 Li2O 14.81  15.00  15.10  14.78  15.01  14.86  14.89  14.98  CaO— — — — — — — — Al2O3 3.29 2.00 3.40 1.16 3.33 1.17 3.31 2.00 P2O5 3.253.50 3.30 3.27 3.30 3.28 3.27 4.32 MgO 1.00 — — — 1.01 — 1.00 0.11 TiO2— — — — — — — — ZrO2 — 2.00 0.60 1.99 — 2.00 0.80 0.60 ZnO — 2.00 2.001.40 — 1.00 — 2.44 SnO2 — — — — — — — — CeO2 1.70 0.60 1.70 0.60 0.510.60 0.50 0.63 La2O3 — 1.00 0.30 — — — — 0.30 V2O5 0.10 0.04 0.10 — — —— — BiO3 — — — — — — — — Ta2O5 — — — — — — — — Tb4O7 0.50 0.40 0.50 0.500.51 0.50 0.50 0.50 Plateau (° C.) 600- 620- 620- 570- 600- 580- 540-580- 700 710 690 700 720 720 720 700 Main crystal LS LS LS LS LS LS LSLS phase Example 9 10 11 12 13 14 15 16 Component wt.-% wt.-% wt.-%wt.-% wt.-% wt.-% wt.-% wt.-% SiO2 64.21  64.96  64.90  68.00  68.8 69.00  66.81  64.93  K2O 3.26 3.51 3.50 4.20 2.00 1.00 2.15 3.00 NaO — —— — 2.00 — — — Li2O 13.34  13.49  13.24  15.00  15.00  14.00  13.72 13.45  CaO — — — — — — — 1.00 B2O3 1.19 — — — — — — — Al2O3 2.96 3.093.08 5.00 3.00 3.00 3.00 2.00 P2O5 3.16 2.95 3.28 3.80 5.00 3.00 3.103.00 MgO — — — — — — — — TiO2 — — — — — — — — ZrO2 9.29 12.00  11.00  —— 9.00 8.60 8.00 ZnO — — — 4.00 4.20 — — — SnO2 — — — — — — — — CeO21.78 — 0.50 — — 0.50 0.60 1.90 Y2O3 — — — — — — 1.52 — V2O5 0.12 — — — —— — 0.12 MnO2 0.20 — — — — — — — Ta2O5 — — — — — — — 2.00 Tb4O7 0.49 —0.50 — — 0.50 0.50 0.50 Er2O3 — — — — — — — 0.10 Plateau 620- 650- 630-560- 530- 650- 660- 700- (° C.) 770 800 760 780 730 755 780 810 Main LSLS LS LS LS LS LS LS crystal phaseLS lithium metasilicateLS lithium metasilicate

Example 17 Examination of Sintering Behaviour of the CompositionAccording to Example 1

A glass with the composition according to Example 1 was melted andground to a glass powder with an average particle size of 20 μm,relative to the number of particles. This glass powder was provided witha binder customary in the trade and uniaxially pressed to form cylindersat a pressure of 80 MPa. The sintering behaviour of these cylindricalblanks was examined by heat-treating them at different temperatures in afurnace of the Programat® P700 type from Ivoclar Vivadent AG. In eachcase a heating rate of 10° C./min and a holding time of 15 min at therespective temperature were chosen. After that the blanks were cooled toroom temperature and the relative density of the blanks was thendetermined in each case in relation to the true density of thecorresponding dense-sintered lithium disilicate glass ceramic. Theremaining linear shrinkage and from that the enlargement factor to bechosen were calculated from the relative density.

The results for sintering temperatures in the range of from 25 to 870°C. are shown in the following Table II. A pre-sintered lithiummetasilicate glass ceramic blank according to the invention with arelative density of from 74.7 to 75.4% was present at between 600 and700° C.

TABLE II Remaining lin. Enlargement Pre-sintering Relative shrinkagefactor temperature Height Diameter Mass Density density S F_(v) T [° C.]H [mm] D [mm] m [g] ρ [g/cm3] ρ/ρ₀ 1-(ρ_(r))^(1/3) 1/(1-S)  20° C.12.231 16.020 3.9556 1.604 64.7% 13.5% 1.156 200° C. 12.303 16.0433.9847 1.602 64.6% 13.6% 1.157 300° C. 12.320 16.043 3.9861 1.601 64.5%13.6% 1.157 400° C. 12.201 16.036 3.9287 1.594 64.3% 13.7% 1.159 500° C.11.628 16.030 3.8769 1.652 66.6% 12.7% 1.145 550° C. 11.703 15.7603.9387 1.725 69.6% 11.4% 1.129 570° C. 11.820 15.245 3.9567 1.834 73.9%9.6% 1.106 600° C. 11.704 15.200 3.9362 1.853 74.7% 9.3% 1.102 630° C.11.570 15.206 3.9273 1.869 75.4% 9.0% 1.099 660° C. 11.562 15.260 3.95291.869 75.4% 9.0% 1.099 700° C. 11.682 15.170 3.9469 1.869 75.4% 9.0%1.099 750° C. 11.600 15.050 3.9134 1.896 76.5% 8.6% 1.094 800° C. 11.17414.830 3.8838 2.012 81.1% 6.7% 1.072 850° C. 10.384 13.940 3.8706 2.44298.5% 0.5% 1.005 870° C. 10.402 13.922 3.9172 2.474 99.8% 0.1% 1.001

In FIG. 1 a graphic representation of the relative density as a functionof the pre-sintering temperature is shown.

In FIG. 2 the calculated enlargement factor is plotted against thepre-sintering temperature. It can be seen from this that the enlargementfactor surprisingly remains substantially constant in the range of from600 to 700° C. and the curve forms a plateau. Thus, when a pre-sinteringis applied in this range, a blank according to the invention can beproduced for which a very precise specification of the enlargementfactor to be chosen is possible.

Example 18 Examination of Sintering Behaviour of the CompositionAccording to Example 8

The sintering behaviour of the composition according to Example 8 wasexamined analogously to Example 17. A glass with the compositionaccording to Example 8 was melted and ground to a glass powder with anaverage particle size of 15 μm, relative to the number of particles.This glass powder was pressed to form cylinders as described previously.The sintering behaviour of these cylindrical blanks was examined byheat-treating the testpieces at different temperatures in a furnace ofthe Programat® P700 type from Ivoclar Vivadent AG. In each case aheating rate of 10° C./min and a holding time of 2 min at the respectivetemperature were chosen. After that the blanks were cooled to roomtemperature and the relative density of the blanks was then determinedin each case in relation to the density of the correspondingdense-sintered lithium disilicate glass ceramic. The remaining linearshrinkage and from that the enlargement factor to be chosen werecalculated from the relative density.

The results for sintering temperatures in the range of from 25 to 870°C. are shown in the following Table III. A pre-sintered lithiummetasilicate glass ceramic blank according to the invention with arelative density of from 74.4 to 75.1% was present at between 580° C.and 700° C.

TABLE III Remaining lin. Enlargement Pre-sintering Relative shrinkagefactor temperature Height Diameter Mass Density density S F_(v) T [° C.]H [mm] D [mm] m [g] ρ [g/cm3] ρ/ρ₀ 1-(ρ_(r))^(1/3) 1/(1-S)  20° C.14.936 14.536 3.9328 1.587 64.8% 13.5% 1.16 200° C. 14.865 14.590 3.93151.582 64.6% 13.6% 1.16 300° C. 14.824 14.648 3.9323 1.574 64.2% 13.7%1.16 400° C. 14.705 14.726 3.9275 1.568 64.0% 13.8% 1.16 500° C. 14.55414.806 3.9281 1.568 64.0% 13.8% 1.16 520° C. 14.474 14.806 3.9293 1.57764.4% 13.7% 1.16 540° C. 14.307 14.659 3.9328 1.629 66.5% 12.7% 1.15560° C. 14.147 14.143 3.9113 1.760 71.8% 10.4% 1.12 580° C. 14.05213.939 3.9113 1.824 74.4% 9.4% 1.10 600° C. 14.033 13.975 3.9299 1.82674.5% 9.3% 1.10 650° C. 14.055 13.949 3.9252 1.827 74.6% 9.3% 1.10 700°C. 14.082 13.915 3.9394 1.840 75.1% 9.1% 1.10 750° C. 13.910 13.7383.9369 1.909 77.9% 8.0% 1.09 780° C. 13.792 13.625 3.9415 1.960 80.0%7.2% 1.08 780° C. 13.797 13.630 3.9450 1.960 80.0% 7.2% 1.08 800° C.13.593 13.500 3.9302 2.020 82.4% 6.2% 1.07 825° C. 13.252 13.117 3.93472.197 89.7% 3.6% 1.04 850° C. 12.779 12.701 3.9231 2.423 98.9% 0.4% 1.00870° C. 12.750 12.670 3.9282 2.444 99.7% 0.1% 1.00

In FIG. 3 a graphic representation of the relative density as a functionof the pre-sintering temperature is shown.

In FIG. 4 the calculated enlargement factor is plotted against thepre-sintering temperature. It can be seen from this that the enlargementfactor surprisingly remains substantially constant in the range of from580 to 700° C. and the curve forms a plateau. Thus, when a pre-sinteringis applied in this range, a blank according to the invention can beproduced for which a very precise specification of the enlargementfactor to be chosen is possible.

The same process for determining this range (“plateau”) was used for theother compositions given in Table I.

1. Pre-sintered blank for dental purposes based on lithium metasilicateglass ceramic, wherein the blank has a relative density of from 66 to90% relative to the true density of the corresponding dense-sinteredlithium disilicate glass ceramic.
 2. Pre-sintered blank according toclaim 1, which consists substantially of lithium disilicate glassceramic.
 3. Pre-sintered blank according to claim 1, wherein the glassceramic includes lithium metasilicate as main crystal phase and containsmore than 20 vol.-% lithium disilicate crystals.
 4. Pre-sintered blankaccording to claim 1, wherein the lithium metasilicate glass ceramiccontains at least one of the following components: Component wt.-% SiO₂50.0 to 80.0 Li₂O 6.0 to 20.0 Me(I)₂O 0 to 10.0, in particular 0.1 to10.0 Me(II)O 0 to 12.0, in particular 0.1 to 12.0 Me(III)₂O₃ 0 to 8.0,in particular 0.1 to 8.0 Me(IV)O₂ 0 to 8.0, in particular 0.1 to 8.0Me(V)₂O₅ 0 to 8.0, in particular 0.1 to 8.0 Me(VI)O₃ 0 to 8.0, inparticular 0.1 to 8.0 nucleating agent 0 to 8.0, in particular 0.1 to8.0,

wherein Me(I)₂O is selected from Na₂O, K₂O, Rb₂O, Cs₂O or mixturesthereof, Me(II)O is selected from CaO, BaO, MgO, SrO, ZnO and mixturesthereof, Me(III)₂O₃ is selected from Al₂O₃, La₂O₃, Bi₂O₃, Y₂O₃, Yb₂O₃and mixtures thereof, Me(IV)O₂ is selected from ZrO₂, TiO₂, SnO₂, GeO₂and mixtures thereof, Me(V)₂O₅ is selected from Ta₂O₅, Nb₂O₅ andmixtures thereof, Me(VI)O₃ is selected from WO₃, MoO₃ and mixturesthereof, and nucleating agent is selected from P₂O₅, metals and mixturesthereof.
 5. Pre-sintered blank according to claim 1, which has at leasttwo areas which differ by their coloration or translucence. 6.Pre-sintered blank according to claim 1, which has a holder for aprocessing device.
 7. Pre-sintered blank according to claim 1, which hasan interface in the form of a recess, for connection to a dentalimplant.
 8. Pre-sintered blank according to claim 1, which is obtainableby the process in which (a) lithium silicate glass in powder orgranulate form is pressed to form a glass blank, (b) the glass blank isheat-treated in order to prepare a pre-sintered blank based on lithiummetasilicate glass ceramic, wherein the temperature of the heattreatment (i) is at least 500° C., and (ii) lies in a range whichextends over at least 30K, and in which the relative density varies byless than 2.5%.
 9. Pre-sintered blank according to claim 1, which has arelative density which results when (a) powder of a correspondingstarting glass with an average particle size of <50 μm, relative to thenumber of particles, is uniaxially or isostatically pressed at apressure of from 20 MPa to 200 MPa and (b) the obtained glass powdergreen compact is heat-treated for 5 to 60 min at a temperature which (i)is at least 500° C., and (ii) lies in a range which extends over atleast 30K and in which the relative density varies by less than 2.5%.10. Process for the preparation of the blank according to claim 1, inwhich (a) lithium silicate glass in powder or granulate form is pressedto form a glass blank, (b) the glass blank is heat-treated in order toprepare a pre-sintered blank based on lithium metasilicate glassceramic, wherein the temperature of the heat treatment (i) is at least500° C., and (ii) lies in a range which extends over at least 30K and inwhich the relative density varies by less than 2.5%.
 11. Processaccording to claim 10, in which in step (a) at least two lithiumsilicate glasses are used which differ in terms of their coloration ortranslucence.
 12. Process for the preparation of dental restorations, inwhich (i) the pre-sintered blank based on lithium metasilicate glassceramic according to claim 1 is shaped by machining to form a precursorof the dental restoration, (ii) the precursor is substantiallydense-sintered in order to produce the dental restoration, and (iii)optionally the surface of the dental restoration is provided with afinish.
 13. Process according to claim 12, in which the machining iscarried out with computer-controlled milling and/or grinding devices.14. Process according to claim 12, in which (a) lithium silicate glassin powder or granulate form is pressed to form a glass blank, (b) theglass blank is heat-treated in order to prepare a pre-sintered blankbased on lithium metasilicate glass ceramic, wherein the temperature ofthe heat treatment (i) is at least 500° C., and (ii) lies in a rangewhich extends over at least 30K and in which the relative density variesby less than 2.5% in order to obtain the pre-sintered blank based onlithium metasilicate glass ceramic.
 15. Process according to claim 12,in which the dental restorations are selected from crowns, abutments,abutment crowns, inlays, onlays, veneers, shells, bridges andoverstructures.
 16. Method of using Use of the blank according to claim1 to prepare dental restorations comprising crowns, abutments, abutmentcrowns, inlays, onlays, veneers, shells, bridges and overstructures. 17.Pre-sintered blank according to claim 1, wherein the relative density isfrom 68 to 88%, relative to the true density of the correspondingdense-sintered lithium disilicate glass ceramic.
 18. Pre-sintered blankaccording to claim 1, wherein the relative density is from 70 to 86%,relative to the true density of the corresponding dense-sintered lithiumdisilicate glass ceramic.
 19. Pre-sintered blank according to claim 3,wherein the glass ceramic contains more than 25 vol.-% lithiumdisilicate crystals.
 20. Pre-sintered blank according to claim 3,wherein the glass ceramic contains more than 30 vol.-% lithiumdisilicate crystals.
 21. Pre-sintered blank according to any claim 9,wherein the powder of the corresponding starting glass is uniaxially orisostatically pressed at a pressure of from 40 MPa to 120 MPa and theobtained glass powder green compact is heat-treated for 10 to 40 min ata temperature which is at least 540° C., and lies in a range whichextends over at least 50K and in which the relative density varies byless than 2.0%
 22. Pre-sintered blank according to any claim 9, whereinthe powder of the corresponding starting glass is uniaxially orisostatically pressed at a pressure of from 50 MPa to 100 MPa and theobtained glass powder green compact is heat-treated for 15 to 30 min ata temperature which is at least 580° C., and lies in a range whichextends over at least 70K and in which the relative density varies byless than 1.5%.
 23. Process for the preparation of the blank accordingto 10, wherein the temperature of the heat treatment is at least 540°C., and lies in a range which extends over at least 50K and in which therelative density varies by less than 2.0%.
 24. Process for thepreparation of the blank according to 10, wherein the temperature of theheat treatment is at least 580° C., and lies in a range which extendsover at least 70K, and in which the relative density varies by less than1.5%.